The present invention relates generally to a method and system for inductively heating a workpiece, and more specifically to a method and system for inductively heating a workpiece by a plurality of coiled tubing assemblies.
There are many different approaches in heating a “workpiece” (the material to be heated), and the selected approach depends on many considerations, such as the purpose for the heating, size and specifications of the workpiece, power requirements, and time limitations for the heating process. A few commonly known methods for heating a workpiece include the use of electrical resistance, combustion, and induction. The electrical resistance method generally involves the creation of heat by the flow of electric current through a conductor or element of high resistance. A disadvantage of this method is that it is usually limited to heating smaller workpieces or localized areas on larger workpieces due to the large power requirement and lengthy time to wrap wire and heating elements around the workpiece. Another disadvantage is the fact that the conductor itself becomes very hot, thereby increasing the potential for injury during operation. The combustion method generally involves the creation of heat by the exothermic reaction between a fuel and an oxidant. A disadvantage of this method is that it is usually limited to large workpieces that do not need finite temperature control and heat placement. The induction technique creates heat by applying an induced magnetic field around the workpiece that creates resistance (and heat) in the workpiece. There are many advantages of induction heating over other traditional heating techniques, such as quick heating of the workpiece, heating without direct contact between the coil and the workpiece, narrowly focused heat application, consistent and improved heating results, and efficient power consumption.
In general, the basic principles of induction and the techniques for heating a workpiece through an induction method are well known. It is presently thought that the induction technique heats the workpiece by the result of hysteresis and eddy current losses in the workpiece. Thus, magnetic workpieces are easier to heat than non-magnetic workpieces. The induced magnetic field is created by wrapping a coil around the workpiece and supplying a high frequency alternating current by a remote power source to the coil to create an alternating magnetic field around the workpiece. The frequency of the requisite alternating current depends on the workpiece's size, material type, and coupling (interaction between the workpiece and the coil), and the desired penetration depth of the created heat in the workpiece. The coil is typically made of copper tubing (or another material with good conductivity) and is cooled with a fluid such as water. The diameter, shape, and number of turns of the coil influence the efficiency and field pattern of the magnetic field.
Induction heating has a wide range of heating applications, such as surface hardening, melting, brazing, and soldering. In general, dedicated heating coils can be designed and manufactured for small and regularly shaped workpieces. For example, small rigid heating coils have been designed to heat small components in the automobile industry or small pipes in the steel fabrication industry. As the workpiece is increased in size and/or irregularity in its shape, the design and manufacture of an effective heating coil to produce the required temperature and/or heating profile in the container becomes problematic.
Large metal containers are often used to hold a wide variety of toxic chemicals, such as mustard, lewisite, nerve agents, and various commercial chemicals. Once the chemical is removed from the container, the container still has traces of its previous contents that need to be removed. It is known in the industry that large, metal cylindrical containers that previously held toxic or contaminated chemicals can be decontaminated using induction heating with flexible coils, as shown in FIG. 1. Typically, multiple thermocouples are spot welded on a container in predefined locations for the subsequent monitoring of the container's temperature. A layer of thermal insulation is wrapped around the container to mitigate heat losses from the container as it is heated and to protect the heating coils. A long, flexible coiled tubing is then cylindrically wrapped around the shell face of the insulated container. Because the diameter, shape, and number of turns of the coil influence the efficiency and field pattern of the magnetic field, the flexible coil must be properly-positioned on the container to achieve the optimal magnetic field. A power source supplies an alternating current to the coil that creates resistance in the container as a result of the applied and changing magnetic field. The amount of heating in the container increases as the supplied power increases, and as a result, the supplied power can be adjusted to heat the container to the desired temperature and/or heating profile.
Although this method of “flexible coil” induction heating has been used in industry, it suffers from numerous and significant disadvantages. One primary disadvantage is that installation of the flexible coil around the container is not only time and labor intensive but is prone to inconsistencies. Each wrap of the flexible coil must be operatively positioned next to the adjacent wrap to create an effective magnetic field. This positioning includes not only the distance between each wrap of coiled tubing, but the pitch (or angle) and tightness of the flexible coil around the container. It is a long and laborious process to individually wrap and position the flexible coil around the container, and the placement and effectiveness of the flexible coil often varies significantly between each container as a result of inconsistencies and installation error. The inconsistent spacing between adjacent coil wraps is illustrated in FIG. 1. Another common problem is that there is no good way to heat the ends of the container with the use of the traditional flexible coil method. In the case of a cylindrical container, the flexible coil can (with enough time and labor) be wrapped around the shell face of the container, but it is very difficult to wrap the end faces of the cylindrical container with flexible coils, and much more difficult to wrap the end face with any type of precision and consistency. Usually, the end faces are not even wrapped. The unwrapped end faces further provides for inconsistent and ineffective heating results of the container, and as a result, the container cannot be efficiently and/or effectively decontaminated.
What is needed is a system and method for inductively heating a workpiece that will significantly reduce coil installation time, provide a more efficient design for power and heating time limitations, provide efficient heating to all desired portions of the workpiece, and provide a more standardized coil spacing for repetitive heating and temperature uniformity in the workpiece.